Ahh..it's been a while since I made the video, so I hope I remember what I said. Generally, the issue with the 5 Whys is repeatability. If you don't get the same answer regardless of who uses it, you should question the results. For the 5 Whys, it will generally still yield good results on simple problems, and if you are wrong, you just PDCA the heck out of it. But when you want to be really sure, you need to get increasingly scientific using objective studies and formal data collection. Flow charts have some of the same characteristics, but less so. Pareto charts are more scientific because you are taking actual counts/measurements, but there is still some inherent repeatability issues in selecting categories. Run charts get more precise. FMEA can help isolate problems with more structure. A decision tree can guide act sort of like a 5 Whys tree, but with data added at each of the decision points. Control charts are even more scientific. ANOVA and other statistical analysis is even better. As you can imagine, the more scientific and precise you want to get, the more skill and resources you need to invest.